This invention relates to ink jet printers and deals more particularly with an improved apparatus or hydraulic circuit and related method for dynamically varying the pressure of writing fluid supplied to an ink jet printer head having an electrically energizable, activating element such as a piezoelectric
In an ink jet printer a receiving surface on which a graphic is to be created is moved relative to one or more ink jet printer heads in a line scanning fashion. As each printer head moves along a scan line, it moves past a succession of points on the line in relation to each of which the printer head may eject a drop of a writing fluid such as ink, paint, pigmented ink, etc. which lands on and prints a dot at the position. In one type of printer head the head is actuated for each potential print point on the scan line, to eject a drop of ink for each such position, and then the drop is electrostatically controlled during its flight from the printer head to the receiving surface to either direct it onto the receiving surface or away from the receiving surface depending on whether the scan line point in question is to be printed or not. In such a printer head the actuation frequency, or the time between successive actuations, is dependent on the speed of the printer head along the scan line. That is, the actuation frequency, or the time between successive actuations, will change if changes are made in the speed of the printer head relative to the receiving surface.
In another type of printer head, referred to as a "drop-on-demand" printer head, as the printer head is moved along a scan line it is actuated to produce a drop of ink only for those potential print positions along the scan line onto which the printing of dots is wanted. Therefore, the amount of time elapsing between successive actuations is dependent not only on the speed of the printer head relative to the receiving surface, but also on the pattern in accordance to which dots are to be printed along the scan line.
In either type of printer head described above, after a drop is ejected from the head, it travels for some distance in free flight from the printer head to the receiving surface along a trajectory path dependent on the velocity at which the drop is ejected. Changes in the ejected velocity therefore change the location at which a drop strikes the receiving surface and are quite undesirable.
Another significant factor contributing to the generation of quality graphics relates to the consistency of the size of printed dots along the scan line as the actuation frequency of the ink jet head changes. For good printing, all ejected drops should be of substantially the same volume so that all dots printed on the receiving surface by the separate drops are of substantially consistent size.
Because of fluid and mechanical dynamics involved in the actuation of a printer head, including resonances and other phenomena, the ejected drop velocity and volume varies widely in many printer heads with the changes in the actuation frequency or the time elapsing between successive actuations. This may be somewhat troublesome in the use of electrostatically deflected printer heads in the cases where the printer head is moved at different speeds relative to the receiving surface. It is, however, particularly troublesome in the case of "drop-on-demand" printer heads in which the inherent operation of the printer involves a wide range in the elapsed time occurring between successive pulses. That is, while scanning a line during one portion of the line, the printer head may be actuated to print the dot at every potential print point, in which case a very short elapsed time occurs between successive actuations, and along other portions of the line, the printer head may be actuated to print the dot only at some occasional potential print points in which case the time elapsing between successive actuations is considerably lengthened. In the case of large volume "drop-on-demand" ink jet printer heads which produce dots having a printed size ranging from 0.02 inches to 0.05 inches or larger, the fluid and mechanical dynamics tend to limit the rate at which drops having substantially the same volume and constant velocity are ejected from the printer head. Consequently, the receiving surface area coverage per unit time is generally reduced to compensate for printer head and ink supply limitations to produce quality graphics generated by printing dots of substantially the same size.
In a co-pending patent application entitled "METHOD AND APPARATUS FOR DRIVING AN INK JET PRINTER", U.S. Ser. No. 634,499, filed July 26, 1984, now U.S. Pat. No. 4,562,445 and assigned to the same assignee as the present invention, a driving circuit for an ink jet printer head is disclosed for causing the printer head to eject drops at a constant velocity despite changes in the time elapsing between successive actuations to compensate for the various system resonances of the ink jet printer head and its associated writing fluid supply system at the frequencies which might otherwise represent nonproductive dot generation by the ink jet printer head.
Although the above-described driving circuit overcomes substantially the problems of nonproductive dot generation due to fluid and mechanical dynamics, the maximum dynamic range over which dots are produced is further limited by the drop in the pressure of ink within the ink jet head piezo cavity. At higher dot generation rates the volume of an ejected drop and consequently, the size of the printed dot associated with the drop is generally not equal to the volume of an ejected drop and the size of the printed dot produced at lower dot production rates. The limited dynamic range of dot production is most noticeable in periods of very rapid successive actuations of the ink jet head after a period of no or relatively low dot production rates. One reason is that the ink jet head piezo cavity is ink starved at the higher dot production rates because ink is ejected from the head faster than it is supplied from the ink source. This is especially troublesome during transitions from low to high dot production rates because the dots printed are not of substantially consistent size due to the varying ink volume in the ejected ink drops and the quality of the graphic produced is degraded.
The object of the present invention is, therefore, to provide a hydraulic circuit, particularly useful with "drop-on-demand" printer heads, but also useful with electrostatically deflected ink jet printer heads, for dynamically varying the pressure of ink supplied to an ink jet printer head from an ink supply.
Another object of the invention is to provide a hydraulic circuit to supply ink to an ink jet printer head so that drops of a substantially constant volume are ejected despite changes in the time elapsing between successive actuations.